A brake pedal mechanism is usually employed to effectuate braking of a motor vehicle through its braking system. The braking system typically includes a brake booster which supplements the braking force provided by the vehicle operator, which in turn operates a hydraulic master cylinder for pressurizing fluid and the brake lines and applying a braking force to the wheels of the vehicle via individual wheel brakes. The brake pedal mechanism is typically a simple lever, wherein a pedal arm has the brake pedal at one end while the opposing end is pivotally connected to the vehicle frame. An input rod leading to the brake booster is connected to the pedal arm, and based on the position of its connection to the pedal arm, the lever action of the pedal arm increases the output force generated by the input force on the brake pedal, i.e. forms a force ratio of the output force divided by the input force.
One common problem with these typical braking mechanisms is that performance of the brake system often ends in a compromise between having enough force capability to stop the vehicle under mandated failure mode conditions and not having too much pedal travel to maintain good pedal feel. That is because to increase the force ratio the distance the pedal must travel must be increased. If a vehicle is marginal in meeting a deceleration requirement with a given brake pedal input force, a typical step is to increase the brake pedal ratio, resulting in greater braking force and vehicle deceleration. Unfortunately, increasing the force ratio increases the pedal input travel required to meet the same rate of deceleration.
Some designs have attempted to address the problem by providing a multiple link brake pedal which lowers the pedal travel initially, when the pedal is at low stroke and relatively low force values. The concept is that an increase in the force required in this range is acceptable since initial forces are low. Unfortunately, the nature of the linkages results in an overshoot of the ratio required at failed power conditions. This also results in some of the pedal stroke gained initially being lost during the remainder of the pedal travel. That is, the force ratio will steadily increase with pedal travel, but then quickly drops off as pedal travel increases further towards a fully extended position. Thus, the brake system parameters become extremely critical since it must be assured that the failure conditions occur only in the pedal travel zone which provide an adequate force ratio.
Accordingly, there exists a need to provide a simple brake mechanism for use with the braking system in a motor vehicle which does not have too much pedal travel to maintain good pedal feel, but yet provides a sufficient force ratio under failure mode conditions to provide sufficient force capability to stop the vehicle. Ideally, such a brake mechanism also eliminates the unwanted drop off in force ratio as the pedal travel increases towards a fully extended position.